Rockwell International S 2R N4913X accident near Brawley, California

Probable cause

The partial loss of engine power due to the overstress separation of three supercharger drive gear teeth.

NTSB analysis

The commercial pilot reported that, while conducting low-level maneuvering over a field during the agricultural application flight, the engine lost partial power . The pilot leveled the wings, and the engine momentarily regained some power; however, shortly after, it started to lose power again. The pilot subsequently initiated an off-airport landing to a field. During the landing roll, the propeller struck the ground, and the landing gear were torn off. Both wings, the engine mounts, and the bottom of the fuselage sustained substantial damage.

Postaccident examination of the engine revealed that three of the supercharger drive gear teeth had broken, one of which was found in the engine oil sump. Examination of the gear teeth fracture surfaces revealed features and topographies consistent with bending overstress separations. Contact patterns and pitting were noted on the pressure faces of the remaining intact gear teeth, which were indicative of heavy loading. No preexisting cracks were found. The thrust bearing exhibited damage consistent with a propeller strike.

A representative from an engine overhaul facility that overhauls the accident engine make and model (but did not overhaul the accident engine) reported that it is unlikely that the gear teeth fractured during the accident sequence given that one of the separated gear teeth was found in the oil sump and not in the supercharger blower section because typically when a gear tooth separates during flight, the oil flow will carry the broken fragment down into the sump. The representative reported that the gear teeth usually break due to either a sudden engine stoppage or fatigue or stress cracking. The fatigue or stress cracking can often be the result of a propeller strike and, if left unchecked, can result in the eventual separation of the affected gear teeth. He stated that it is possible for an engine to continue operating relatively normally with only one gear tooth missing; however, multiple broken gear teeth would result in a decrease in power of the supercharger section.

Maintenance records were not made available for review during the investigation; therefore, it could not be determined if a propeller strike or sudden engine stoppage was reported before the accident.

Text from the NTSB. Open the NTSB docket for WPR17LA174 for the full record.